From the viewpoint of global environmental conservation, a constant and important issue in the automotive industry in order to reduce CO2 emissions is to improve the fuel consumption of automobiles by reducing the weight of their bodies while the strength of the bodies is maintained. To achieve a reduction in the weight of automobile bodies while their strength is maintained, it is effective to increase the strength of steel sheets used as the materials of automobile parts to thereby allow the thickness of the steel sheets to be reduced. Many automobile parts made of steel sheet materials are formed by press forming, burring, etc. Therefore, high-strength steel sheets used as the materials of automobile parts are required to have, in addition to desired strength, high formability.
In recent years, ultra-high-strength steel sheets having a tensile strength TS of over 1,180 MPa are increasingly used as the materials of frames of automobile bodies. However, these ultra-high-strength steel sheets are difficult to form, and it is difficult to use conventional press forming without any modification, so that processing including mainly bending, such as roll forming, is often used. Therefore, bendability is one of the most important properties when over 1,180 MPa grade ultra-high-strength steel sheets are used. One of the important properties of the materials of automobile parts is anti-crash property. At the time of the collision of an automobile, parts formed of steel sheets are subjected to a strain rate as high as 103/sec. Therefore, automobile parts such as pillars, members, and bumpers are required to have anti-crash property sufficient for ensuring the safety of an occupant in case of collision during driving of the automobile. Specifically, it is necessary to use high-strength steel sheets having anti-crash property, i.e., having a high ability to absorb collision energy even when the steel sheets are subjected to a high strain rate at the time of collision as descried above, to thereby ensure the collision safety of the automobile.
To address these requirements, Patent Literature 1, for example, discloses a technique for an ultra-high-strength steel sheet having a TS of 1,180 MPa grade and excellent in formability and anti-crash property. Patent Literatures 2 and 3 disclose techniques for high-strength steel sheets having good bendability, good spot weldability, etc.